Thanks for your response. Yes. The upgrade to CC is the solution. I downloaded a trial of CC just to test it out. It works fine: both the Edit -> Paste and the Edit -> Paste mocha mask modes.
By the way, it only seemed to happen when I had keyframes rotosplines. Mask shapes with no keyframes translated fine when brought into AE 5.5. A moot point at this point.
Thanks again for your input. Happy roto-ing!
Changgeon